Format checker deepcopy to shallowcopy

[p1c2u]

Should improve the performance. Related to #289

[codecov]

Codecov Report

Merging #291 (a16162f) into master (501694c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   96.74%   96.74%           
=======================================
  Files         122      122           
  Lines        2429     2429           
=======================================
  Hits         2350     2350           
  Misses         79       79           

Impacted Files	Coverage Δ
openapi_core/unmarshalling/schemas/factories.py	100.00% <100.00%> (ø)

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